Discussion

Of the 21 hydrogen-deficient carbon stars observed by Hipparcos, 20 have either null or negative parallaxes (7 stars) or less than 1.5 positive parallaxes (13 stars). None of these 13 positive parallaxes are significant. Indeed, if bias effects such as the Lutz-Kelker statistical bias correction (Lutz & Kelker 1973; Koen 1992) are taken into account, these very weak positive values are most likely over-estimates of the true parallax for each star.

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Table 1: Hipparcos data for 21 hydrogen-deficient carbon stars

	Star	Type	Number
			(HIP)	(J1991.25)		(mas)	(masyr)		(mas)	(masyr)
	XX Cam	HdC	019340	04 08 38.75	+53 21 39.4	-1.04	1.09	-3.46	0.90	0.74	0.71
	SU Tau	RCB	027465	05 48 58.68	+19 04 36.4	3.32	3.57	1.17	5.24	6.16	3.28
	BD +102179	eHe	052133	10 38 55.24	+10 03 48.5	0.49	-12.05	-3.18	1.61	1.83	1.55
	UW Cen	RCB	062071	12 43 17.20	-54 31 40.7	1.27	-7.87	-12.64	2.81	3.26	3.89
	Y Mus	RCB	063911	13 05 48.20	-65 30 46.6	8.02	-4.14	0.08	1.93	1.60	1.70
	HD 124448	eHe	069619	14 14 58.64	-46 17 19.3	0.59	-7.54	1.01	1.75	1.20	1.30
	S Aps	RCB	074179	15 09 24.55	-72 03 45.2	-0.89	-5.45	-0.76	1.56	1.37	1.68
	HD 137613	HdC	075694	15 27 48.32	-25 10 10.1	-1.36	-1.84	-8.11	1.32	1.29	0.94
	R CrB	RCB	077442	15 48 34.42	+28 09 24.4	0.54	-2.10	-11.52	0.72	0.37	0.49
	HD 148839	HdC	081254	16 35 45.80	-67 07 36.7	1.47	-5.31	-3.20	1.11	0.72	0.88
	HD 160641	eHe	086605	17 41 51.58	-17 53 48.5	-1.20	-1.64	1.57	2.14	2.28	1.55
	RS Tel	RCB	089739	18 18 51.22	-46 32 53.4	3.00	-8.10	-5.49	2.98	3.52	2.27
	HD 168476	eHe	090099	18 23 14.66	-56 37 44.1	-0.65	0.04	-9.08	1.30	1.22	0.87
	HD 173409	HdC	092115	18 46 26.63	-31 20 32.1	1.95	-0.05	-4.45	1.57	1.97	1.19
	V CrA	RCB	092207	18 47 32.31	-38 09 32.3	1.18	-6.03	-4.83	2.77	4.12	2.65
	HD 175893	HdC	093181	18 58 47.29	-29 30 18.0	1.38	2.26	-3.36	1.42	1.71	1.05
	SV Sge	RCB	093987	19 08 11.78	+17 37 41.2	0.52	0.82	6.65	3.56	3.05	2.92
	RY Sgr	RCB	094730	19 16 32.76	-33 31 20.3	0.36	10.33	-0.41	1.12	1.15	0.62
	HD 182040	HdC	095289	19 23 10.07	-10 42 11.6	0.00	7.11	1.72	0.98	0.80	0.50
	V482 Cyg	RCB	098411	19 59 42.58	+33 59 28.0	-8.96	-4.84	-12.21	3.81	3.06	4.25
	U Aqr	RCB	108876	22 03 19.70	-16 37 35.3	3.58	3.03	-2.53	2.55	3.19	1.40

The RCB star Y Mus is apparently detected at = 8.02 1.93 mas, ie, a 4.2 detection. Even a detection at this level is marginal for individual usefulness according to Koen (1992). However, even if the parallax was correct, the result is implausible for a RCB star. This parallax measurement corresponds to a distance of 126 30 pc. At 9.3 and (B-V) 0.6 (Lawson et al. 1990; see table 21), this would place the star at 3.8 and therefore near the main-sequence. Y Mus is spectroscopically similar to galactic RCB stars such as RY Sgr and R CrB, and the LMC RCB stars W Men and HV 12842. These are all low-gravity stars, with log 0-1, which must infer an of at least -3 or -4.

There is little likelyhood that Y Mus has been misidentified with another star in the surrounding field. The star has no near-neighbours of similar magnitude and the Hipparcos coordinate is accurate. The star was noted by ESA to be double or else affected by surrounding nebulosity (van Leeuwen, private communication). Either effect would render the parallax unreliable. Binarity would be surprising; none of the RCB stars or related objects are known to be in binary systems, which may be an indicator of their previous evolution. However, a number of RCB stars are known or suspected to be surrounded by extensive nebula (eg, UW Cen; see Pollacco et al. 1991). But if either binarity or nebulosity is the correct explanation, Y Mus is unique in the class in showing such a highly distorted parallax.

We have attempted a mean parallax analysis for the 21 stars and derive -3. The mean result is approximate to the expected value but is subject to large error and therefore must be interpreted with caution, eg, the sample size is small, the mean parallax is small ( = 0.65 mas), and there is uncertainty about the range of in these types of stars.

Table 2 lists cool hydrogen-deficient carbon stars observed by Hipparcos for which we have reliable estimates of (Lawson et al. 1990). In addition to their galactic coordinates (, b) and heliocentric radial velocity (RV), we list galactic distances (XYZ) with respect to the Sun and space velocities (UVW, with 1 uncertainties) derived from the Hipparcos proper motions. The radial velocities for the RCB/HdC stars were mainly sourced from Lawson & Cottrell (1997), with the exception of SU Tau (Drilling & Hill 1986) and XX Cam (Herbig, unpublished). A 0.5 kms uncertainty (1 ) in the radial velocity has been assumed for each star, which also contributes to the space motion uncertainty. For each star the distances and space velocities were calculated assuming either = -5 or -3, which should encompass the likely range of luminosity for these stars. We also derive a theoretical parallax for each star at each luminosity.

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Table 2: Galactic distances, radial and space velocities, and theoretical parallaxes for cool hydrogen-deficient carbon stars

	Star		b		X	Y	Z	RV	U	V	W
																(mas)
	RCB stars
	S Aps	313	-12	-5	3.9	-4.2	-1.2	-75	-144	-53	75	31	27	44		0.17
				-3	1.6	-1.7	-0.5		-87	11	39	12	11	17		0.43
	U Aqr	39	-50	-5	6.6	5.4	-10.1	98	-44	-130	-227	160	84	120		0.076
				-3	2.6	2.2	-4.0		12	-30	-136	64	34	48		0.19
	UW Cen	302	8	-5	2.9	-4.7	0.8	-16	-150	-132	-338	74	47	101		0.18
				-3	1.2	-1.9	0.3		-65	-45	-137	29	19	41		0.45
	V CrA	358	-16	-5	7.4	-0.3	-2.1	-8	19	-248	133	38	107	138		0.13
				-3	2.9	-0.1	-0.8		3	-97	54	15	42	54		0.33
	R CrB	45	51	-5	0.6	0.6	1.0	22	60	-42	18	3	2	2		0.77
				-3	0.2	0.2	0.4		30	-11	18	1	1	1		1.9
	Y Mus	305	-3	-5	3.8	-5.5	-0.3	32	-90	-100	9	42	29	54		0.15
				-3	1.5	-2.2	-0.1		-25	-55	2	16	11	21		0.38
	RY Sgr	5	-20	-5	1.6	0.1	-0.6	-21	-48	22	-68	3	6	8		0.59
				-3	0.6	0.0	-0.2		-30	8	-22	1	2	3		1.5
	SU Tau	189	-4	-5	-3.8	-0.6	-0.3	37	-39	-21	64	10	77	101		0.26
				-3	-1.5	-0.2	-0.1		-38	-12	24	4	31	40		0.65
	RS Tel	348	-14	-5	5.9	-1.3	-1.5	7	-11	-252	144	24	75	96		0.16
				-3	2.4	-0.5	-0.6		0	-100	57	10	30	38		0.40
	HdC stars
	XX Cam	150	1	-5	-1.5	0.9	0.0	9	-22	-18	-15	3	5	6		0.56
				-3	-0.6	0.4	0.0		-13	-4	-6	1	2	2		1.4
	HD 137613	342	25	-5	1.5	-0.5	0.8	71	62	-78	-9	6	9	8		0.56
				-3	0.6	-0.2	0.3		61	-43	15	2	4	3		1.4
	HD 148839	322	-13	-5	2.3	-1.8	-0.7	-12	-56	-64	30	8	9	11		0.33
				-3	0.9	-0.7	-0.3		-28	-21	14	3	4	4		0.83
	HD 173409	4	-13	-5	5.1	0.4	-1.2	-59	-60	-106	-30	11	33	45		0.19
				-3	2.0	0.1	-0.5		-58	-45	-4	5	13	18		0.48
	HD 175893	7	-15	-5	3.4	0.4	-0.9	56	44	-32	-70	8	20	27		0.28
				-3	1.4	0.2	-0.4		50	-9	-37	3	8	11		0.70
	HD 182040	27	-12	-5	1.2	0.6	-0.3	-35	-52	9	-30	2	3	5		0.71
				-3	0.5	0.3	-0.1		-39	-6	-7	1	1	2		1.8

We have not produced XYZ, UVW values for the 4 eHe stars listed in Table 1 due to the large variation in across the eHe-star sequence (eg, Drilling 1986; see fig. 1) and the large and uncertain bolometric correction required for these high-temperature stars.

Some of the results of Table 2 are similar to those obtained previously, but the Hipparcos-derived UVW velocities are new. Drilling (1986) showed the distribution of some of the cooler RCB/HdC stars and most of the known eHe stars in the -b plane and the RV- plane; both planes showing a distribution indicative of a bulge population. Lawson et al. (1990) and Lawson & Cottrell (1990) showed a larger sample of cool hydrogen-deficient carbon stars in the X-Y and X-Z planes, respectively, assuming = -5. These show a known population skewed between = 300-360, and avoiding much of the obscuration of the galactic disk.

Assuming = -5, the U-velocity dispersion of the RCB/HdC stars listed in Table 2 is similar to that reported for the eHe stars by Drilling (1986). Figure 1 shows these stars in the U-velocity/galactic longitude plane. Considering only those stars that lie within 60 of the galactic centre, the U-velocity dispersion of the RCB/HdC stars is 65 kms (1 ), compared to 80 kms (1 ) for the eHe stars. The small difference in the velocity dispersion may be accounted for by the eHe star sample in Figure 1 being a largely complete representation of known eHe stars whereas only the brightest RCB/HdC stars are plotted, but it may simply be a function of the small sample sizes. The velocity and spatial distribution of both these types of stars is similar to that of other predominantly bulge populations, eg, compact planetary nebulae (Drilling 1986).

 
Figure 1: U radial velocity (positive towards the galactic centre) versus galactic longitude for cool hydrogen-deficient carbon stars (open circles) and extreme helium stars (filled circles; data from Drilling 1986). The velocity and spatial distribution of these types of stars is discussed in the text.

With the exception of U Aqr and UW Cen, the W-velocity for the RCB/HdC stars shows a dispersion (60 kms; 1 ) similar to that of the U-velocity. U Aqr is a halo RCB star, with a Z-distance of -10 kpc. The high W-velocity of -227 kms is not unusual for a halo object. UW Cen is only 800 pc above the galactic plane, unexceptional for the RCB/HdC stars listed in Table 2, yet has a W-velocity of -338 kms. UW Cen may be another halo RCB star, seen transitting the galactic plane. However, this velocity does have large uncertainty and the conclusion is sensitive to the adopted absolute magnitude.

The theoretical parallaxes for R CrB, RY Sgr, XX Cam, HD 137613 and HD 182040 should have been detected by Hipparcos, even if with considerable uncertainty, if these stars were = -3. Yet none of these stars returned a statistically significant parallax.

Thus we must await the next generation of parallax/proper motion engines to reliably characterise the distances, proper motions and luminosities of these types of stars. At least the Hipparcos measurements support the results obtained for the LMC hydrogen-deficient stars; the galactic RCB/HdC stars must also be high-luminosity objects.

© Copyright Astronomical Society of Australia 1997